The Sign of q

[Shimona_Gupta_3I]

I know that when heat is given off by the reaction we have a negative q value because the reaction is exothermic, however I am confused as to how we get the negative sign for q. The equation for q = g(Csp)(tf-Ti) however this equation does not result in a negative value for q, so how do we get q to be negative?

[Sanjana Tottemp 3C]

Hello,


You are able to get a negative value for q when delta T or the change in temperature is negative which can happen when the initial temperature is larger than the final temperature, indicating that heat was released causing the temperature to drop, making it exothermic.

Hope That Helps!

[Nikta Seyedin 3D]

Hello,
For the answer to your question and to clarify the concept in the simple terms, it is helpful to think about exothermic when the heat is being released to the surrounding, and endothermic as when heat is absorbed by the system. As a result, during endothermic, when heat is absorbed by the system, the temperature of the system will increase whereas the temperature of the surrounding will decrease. As a result, the final temperature of the system will be larger than the initial temperature of the system. Since the equation is q=m(Csp)(Tf-Ti), then in endothermic reactions the q will be positive. On the other hand, for exothermic reactions, the heat will be released and the temperature of the system will decrease whereas the temperature of the surrounding will increase. As a result, the final temperature of the system will be smaller than the initial temperature of the system. Since the equation is q=m(Csp)(Tf-Ti), then in exothermic reactions, the q will be negative. I hope this helps!